SQUIDs, using a high-temperature superconductor, are used in sensor devices such as an underground resource exploration equipment, a geomagnetic observation equipment, and a non-destructive testing device. Such a sensor device using the SQUID requires liquid nitrogen cooling and low noise conditions.
Conventionally, in the liquid nitrogen cooling process in a SQUID underground resource exploration equipment, geomagnetic observation equipment, non-destructive testing device, and the like, where metal blocks should not be adjacently positioned around a sensor in particular, most of the time, the sensor has been directly immersed in the liquid nitrogen by using a rod-shaped probe rod. Here, low frequency noise, due to vibration caused by bubbles generated as the liquid nitrogen evaporates, adversely affects the measurement accuracy. Thus, a technique for preventing the vibration is required.
In order to control the vibration, Cited Literature 1 introduces a method of performing a control to prevent the bubbles from being generated as the liquid nitrogen evaporates, by providing a rough surface member on an inner bottom surface portion of an insulating container and the like. However, with this conventional method, large bubbles are prevented from being generated, but relatively small bubbles are generated, and thus the low frequency noise due to the vibration of the sensor still remains. Thus, such vibration needs to be further prevented to achieve more accurate measurement.
Cited Literature 2 introduces a method of forming a concaved recess in a bottom portion of a container and fitting a rod, having a sensor attached to the distal end, in the recess. A material that solidifies in the liquid nitrogen may be provided in the fitting portion for fixing and thus further stabilizing the sensor.
However, this conventional method has a problem that the rod cannot be freely inserted and pulled out while the liquid nitrogen is contained, and thus the sensor replacement and liquid nitrogen refilling cannot be easily performed. Thus, to use the device in an outdoor measurement site, for underground resource exploration, geomagnetic observation, and the like, an improvement is required so that such operations can be easily performed.